1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an internal combustion engine having a canister for absorbing fuel vapor in the fuel tank of a vehicle.
2. Description of the Related Art
In an internal combustion engine, it is known to provide a canister for absorbing fuel vapor in a fuel tank, and after absorption, to allow the fuel in the canister to be purged to the engine in accordance with the engine operating conditions. The purge of fuel from the canister to the engine, however, makes the air-fuel ratio unexpectedly richer, and thus interrelated the purging control and fuel supply control must be carried out.
For example, Japanese Unexamined Patent Publication No. 62-131962 discloses an internal combustion engine having a canister, purge control means, and an oxygen sensor for a feedback control of the air-fuel ratio. In this publication, during the purge time, a 10 percent purge flow control is first carried out while learning a variation in the correction factor of the air-fuel ratio and estimating changes in the correction factor of the air-fuel ratio at a 100 percent purge flow control, and subsequently, the 100 percent purge flow control is carried out simultaneously with a feedback control of the air-fuel ratio using the value estimated for the 100 percent purge.
U.S. Pat. No. 3884204 discloses a fuel vapor control device in which, when the fuel tank is being filled with fuel, a seal is established between the inlet opening of the fuel tank and the filler nozzle and the fuel tank is connected to the canister via a pipe, to prevent fuel vapor from escaping to the atmosphere during the filling of the fuel tank.
Japanese Patent Application No. 61-255745, filed by the same assignee for the present case, discloses an internal combustion engine in which fuel vapor generated during the filling of the fuel tank is absorbed in the canister, and a restriction and a solenoid valve are arranged in parallel in the purge passage between the fuel tank and the canister. The solenoid valve is closed when the engine is first started after the filling of the fuel tank, to allow the fuel to be gradually purged through the restriction, and the solenoid valve is opened after a predetermined time to allow large quantities of the fuel to be purged.
In the above systems, however, the following problems arise. A large quantity of the fuel vapor is generated during the filling of the fuel tank with fuel, so that the air-fuel ratio during the purge just after the filling tends to become very much richer. Also, the rate of purging of the fuel, i.e., the rate of release of the fuel vapor from the canister, has a peak at the initial stage of the purging and decreases with the elapse of time, even when the opening of the purge flow control valve is maintained at a constant value.
In the technique disclosed in the above-described Japanese Unexamined Patent Publication No. 62-131962, no special consideration is given to the fuel vapor generated during the filling of the fuel tank, so that the 10 percent purge flow control is always carried out at the initial stage of each purge, to learn variations of the correction factor of the air-fuel ratio. Therefore, at the initial stage of the purge after filling the fuel tank, the fuel purge flow is restricted by partially closing the purge flow control valve, but the actual fuel purge flow is high, for the above reasons, and thus the air-fuel ratio becomes very rich, with the result that control delays occur and the drivability is deteriorated even after the air-fuel ratio is brought to the lean side.
In the technique in the above Japanese Application, fuel is gradually purged through the restriction by closing the solenoid valve, to prevent an overrich air-fuel ratio at the initial stage of the purge. Nevertheless, the air-fuel ratio becomes overrich and the drivability is deteriorated. To overcome this problem, the cross sectional area of the restriction must be made very small, but this means that a long time is needed to complete the purging of the fuel from the canister. Further, since the canister has a restricted absorption capacity, the fuel must be purged when the fuel is already absorbed thereon and the absorbing capacity recovered as soon as possible for the next absorption cycle. Therefore if long time is needed to complete the purging of fuel from the canister, the absorption capacity for absorbing the subsequently generated fuel vapor is reduced.
Further, in the above described arts, parallel passages or an additional valve must be provided for controlling the fuel purge flow step by step, from a large quantity to a small quantity, resulting in a complicated purging arrangement.